Chilling of Milk

i. Importance

Milk leaves the udder at body temperature containing only a few microorganisms.The number increases rapidly at this temperature, if growth is not checked immediately by chilling the milk. Chilling is necessary after receiving milk at collection/chilling center. Chilled milk can easily and safely be transported without having appreciable deteriorative changes due to microbial growth. Thus, raw milk is chilled to a) limit the growth of bacteria, b) minimize micro-induced changes, and c)maximize its shelf life. However, chilling of milk involves additional expense which increases the cost of processing. Importantly, chilling process does not kill microorganisms nor it renders milk safe for human consumption. It is only a means of checking the growth of microorganisms for a certain period.

 

ii. Effect on Microbial Growth

Generally, milk is cooled immediately after milking to below 10°C. within 4 hours to prevent/retard the multiplication of thermophilic and mesophilic bacteria including disease producing and food poisoning organisms until the milk reaches the dairy.The extent of control of growth of microorganisms is dependent on type of organisms.Staphylococci do not grow below 10°C. Growth stops for most types of B. coli,B. proteus and Micrococci between 0°C and 5°C. If milk is stored cold for too long time, there can be an undesirable increase in psychrotrophic organisms which produce extremely heat resistant lipases and proteases.

The time factor is critical in arresting bacterial growth in fresh milk. As milk from the udder of healthy cows has a low bacterial count. There is a lag phase immediately after milking, for around 4 hours, before bacterial multiplication begins to grow. The quicker milk is cooled, the better the quality and in the ideal condition, milk is cooled immediately after milking to 40C or below and held at that temperature till it is processed. The effect of storage temperature on microbial growth in raw milk is shown in below Table.

Effect of storage temperature on microbial growth in milk

iii. Effect on Keeping Quality of Milk

Fresh raw milk is cooled to 40C to extend its shelf-life (freshness). At this temperature, the activity of enzymes, the growth of microorganisms and metabolic processes are all slowed down. As a result, prolonged holding of chilled milk is bound to cause significant deteriorative alterations in keeping quality of milk. In addition, cooling causes a considerable dissociation of b-casein, calcium and phosphate ions and proteases from the casein micelles. The milk loses its suitability for cheese making, coagulation times are increased and the curd tension of the coagulum is less.

Chemical and biochemical processes are considerably slowed down by cooling.However, milk, which has been stored, sometime has a bitter off-flavour. Enzymes and microorganisms can cause chemical changes which are accompanied by a lower pH value and change in nitrogen-containing compounds. Psychrophilic microorganisms cause proteolysis of casein and, together with enzymes, also that of albumin. Protein breakdown products (polypeptides) are formed. Certain bacteria are responsible for the hydrolysis of fats causing rancid flavour development.Several enzymes such as oxidase, catalase and reductase are active for a long time, even at 0° C. Hence, if the time between milk reception and processing is 2 to 3 days, the storage temperature should be kept between 2° C to 5° C for minimum effect on keeping quality of milk.

 

iv. Effect on Physico-chemical Properties of Milk

The effect of rapid cooling and storage at low temperatures on the physico-chemical properties of the milk components are being discussed below:

•  Failure to rennet/acid coagulation: The failure of casein to coagulate at 2°C either at pH 4.7 or after rennet treatment has been utilized in the development of continuous cheese making process, where the milk is either acidified or renneted at 2°C and the temperature is subsequently raised to about 15.6°C or 30°C to effect coagulation.
• Failure to coagulate at isoelectric point: Milk fails to coagulate at 2° C after adjusting to the isoelectric point (pH) of casein. At 2-3° C there is an increase in the diffusible inorganic salts and a change in the casein micelle structure. Some micellar casein is converted to a non-micellar or soluble form(e.g. b-casein). At 2°C, the pH of the milk has to be reduced to 4.3 to effect complete casein coagulation, whereas at 30°C the recovery of the casein at pH 4.6 was nearly complete. Also the properties of casein obtained by acid precipitation at 2° C and pH 4.3, and at 30°C and pH 4.6 were found slightly different.
•  Increase in viscosity: Storage of milk at 20 to 5°C, both raw and pasteurized caused an increase in the viscosity of the product which may be related to changes in the protein system, since viscosity is influenced largely by the colloidal components of milk. Probably, conversion of colloidal calcium partly to soluble form may uncoil the casein micelle. The change in viscosity with storage at low temperature (2 to 5°C) was greatest during the first 24 hours and reaches maximum after about 72 hours.
•  Decrease in cheese curd firmness: The cold aging of milk increased the rennet coagulation time at 30°C. The increased coagulation time was inversely related to the ratio of colloidal calcium-phosphate, and could be reversed by heating to 40°C for 10 minutes or by addition of calcium chloride to the milk prior to cold aging.
•  Increased hydrolytic rancidity: Cold storage of milk below 7°C is associated with an increase in the rate of development of rancidity. Cooling tends to dissociate the casein micelle and increases the total available lipase in the milk system. Subsequent treatment to milk (warming, agitation, etc.) bring lipases into contact with fat globules and liberate free fatty acids to produce rancidity in milk.
•  Increased Foaming: Cold milk foams readily. Milk proteins concentrate in the lamellae of the foam where b-lactoglobulin acts as a surface active agent.Foams are formed by the preferential adsorption of surface active materials at an air-liquid interface with orientation of the material to form an air bubble.
•  Physical structure of fat globules: Crystal structure and size vary as a function of both cooling rate and cooling temperature and regulate the hardness of the milkfat. More fat passes into the solid state by direct cooling than by stepwise cooling. The sensitivity of the fat globule membrane to shear and subsequent release of free fat is greater in milk that has a higher proportion of solid to liquid fat. Thus, milk rapidly cooled, 0-5°C, is more sensitive to shear damage than that is cooled more slowly and in a stepwise manner.

i) Increased clustering of fat: When milk is stored at low temperature (0-5°C),change in the surface characteristic of fat globule membrane results in more rapid creaming due to increased fat globule clustering effected by concentration of “agglutinin” on the fat globule surface.

ii) Migration of substances: Cooling milk to 4°C leads to migration of some membrane proteins, phospholipids, xanthin oxidase, natural copper, etc. from fat globule membrane to milk plasma.

 

v. Equipment and Methods of Chilling

Cooling is the predominant method of maintaining milk quality during collection. The most important factor next to hygienic production of milk is the time between completion of milking and reducing the temperature low enough to restrict bacterial growth. Whatever the method of cooling, the faster the temperature is reduced from 37°C at milking, the better will be the resultant milk quality.Selection of a suitable method and equipment for prompt cooling i.e. chilling milk is dependent upon the available facilities at the moment keeping in view the volume of milk handling and time for cooling and keeping it cold till reaches for processing. Various methods of cooling of milk are described below:

•  Can (container) Immersion: The fresh milk immediately after milking, is placed in a container (preferably metal) which is gently lowered into a tank/trough of cooling water. Cooling of milk will slowly take place and if the water is cold enough, the milk temperature will be reduced low enough to allow the milk to be marketed/processed. The milk inside the cans may be stirred with the help of plunger for uniform quick cooling.
•  Surface Cooler: An improvement of water cooling is a metal surface cooler,where water flows through the inner side and milk flows over the outer surface in a thin layer. A well designed water cooler will reduce milk temperature almost instantaneously. The cooled milk is received below in a receiving trough, from which it is discharged by gravity or a pump.
•  Plate Heat Exchanger: This is the most widely used very effective equipment for chilling of milk by the commercial dairy plants. Several stainless steel plates are mounted on a solid stainless steel frame in which the milk to be chilled and chilling water flow alternatively and counter-currently. The number and size of plates in the exchanger depend upon the capacity of the plant which may vary as per requirement. This method of chilling is more efficient, more hygienic, involves less manual labour and cost effective.
•  Tubular Cooler: This consists of two concentric tubes, inner tube usually carries the milk to be chilled while cold water is passing through the hollow space in between the pipes. The length and diameter of both the tubes are determined according to the capacity of the plant. The flow of the milk and chilled water is in opposite direction, i.e. counter-current. The tubular cooler is efficient,, where milk is not exposed to atmosphere.
•  Bulk Milk Coolers: These consist of a double jacketed vat fitted with a mechanical agitator. It also has provision for circulation of chilled water which comes from the chilled water tank. Normally, milk is chilled and subsequently stored at low temperature until transported to processing units for further processing. Bulk milk coolers are generally installed at chilling centres.
•  Rotor Freeze: Rotor freeze provides spray of chilled water outside the cans obtained by mechanical refrigeration system and passing through the perforated tubes around the neck of the can. With this system, milk temperature is brought down to 10°C from 35°C within 15 minutes.

Brine Cooling: The direct expansion coil is used to cool brine which is then circulated by a pump around the product to be cooled. Brine system of cooling may be of a) brine circulating type, b) brine storage type, and c) congealing-tank type.This system has the advantages of being safe with ammonia and of causing less damage in case of a leakage and the temperature can be easily controlled. It also allows the storing up of refrigeration in the cold brine and therefore, allows heavy refrigerating loads of short duration to be carried with a system having a much smaller compressor than direct expansion system used. The overall thermal efficiency of a brine system is usually less than the direct expansion system on account of the one extra heat transfer and the added radiation losses.

Ice Cooling: Ice, produced by commercial ice plants, is used in some countries to cool milk. The use of ice for cooling is generally fairly expensive and not particularly effective due to the problems in getting an optimum and rapid heat transfer from the liquid milk to the solid ice. Different methods of cooling milk by ice are:

•   In Can Cooling: In this method, ice is placed in a metal container, known as ice gum or ice cone, which is inserted into the can of milk. This permits a more effective heat exchange rate by giving off latent heat of ice and sensible heat of melted water but reduces the volume of milk that can be carried in the milk can. When ice is completely melted in the ice cone and there is no more heat transfer, the water is thrown and fresh ice pieces are put in. The process of cooling milk by this method continues even during transportation from collection centres to processing unit.
•  Direct Addition of Ice: Sometimes cooling of milk is done by direct putting ice into the milk. While this achieves an effective transfer of energy, and reasonably rapid cooling, it has a major disadvantage of diluting the milk with water, which will require removal at subsequent processing or the sale of adulterated milk.
•  Mechanical Cooling: Mechanical refrigeration system is the most effective means of arresting bacterial growth by lowering milk temperature to around 40C. This system of cooling can be utilized in the following manners:
•  Household Refrigerator: This is a practical method for small volume of milk(say from 1 or 2 animals, approx. 5 litres) where the farmers has a refrigerator.The milk in metal container immediately after milking, is placed in a domestic household refrigerator where the milk will slowly cool to the temperature of the refrigerator.
•  Surface/Immersion Cooler: Under direct expansion system, a mechanical refrigerator compressor and condenser (usually air cooled) produces a liquid refrigerant (freon or ammonia) which when passing through an expansion system causes a rapid reduction in temperature.
•  Expansion Bulk Tank: Direct expansion bulk tank, ranging in size from 500 L to 20,000 L, is an energy efficient system of cooling the milk to 40 C within the acceptable period of 4 hours. It is used directly on farms where, medium to large sized herds are milked or at collection/chilling centre.
•  Putting Ice Around Metal Cans of Milk: It is the simplest form of cooling milk in which ice slabs are stacked around the metal cans of milk on the delivery vehicle and the system relies on heat transfer by contact.
• Ice Bank: The ice bank is a widely used for fast cooling of milk. This method of cooling reduces the size of the refrigeration compressor (hence, power requirement) by building up a reserve of ice over a long period. In ice bank,cooling is done through a plate heat exchanger or a surface type cooler with chilled water being the cooling medium. The chilled water is pumped from the ice bank through the heat exchanger and back to the ice bank. Ice banks have considerable flexibility in size and range from a small, self-contained portable unit to a large, using a multiple ammonia compressors, water condensers and associated cooling towers.